A city, town, or county is divided into precincts, each consisting of a voting place. When a person casts their vote in person, they go to their local precinct to cast their vote.
Each precinct has at least 1 GOP precinct committeeman. The precinct committeeman represents the GOP in that precinct. All of the precinct committeemen within the city or county (unit) are members of the unit committee. In Virginia, for example, there are 132 unit committees throughout the state. The position of Precinct Committeeman is either elected or appointed.
A precinct committeeman is the grassroots of the Republican Party. It is also a good place to start for people interested in running for political office.